This invention relates to a method of painting an article formed of a synthetic resin such as an automobile bumper formed of a polyurethane resin.
Current automobiles employ many parts formed of synthetic resins. Various kinds of synthetic resins are used, and particularly polyurethane resins are widely used for relatively large-sized parts represented by bumpers because of excellence and balance in many properties such as moldability, toughness, flexibility and resiliency.
Automobile parts formed of synthetic resins, and particularly ones formed of polyurethane resins, are often painted for improvement of appearance, weather resistance, chemical resistance, and so on. In the conventional painting process for such purpose an indispensable step is coating the surface of the synthetic resin part with a primer. For example, a polyurethane resin base primer containing 0.2-0.5 wt % of carbon black and 5-6 wt % of titanium oxide, a white pigment, is generally used for almost every kind of moldable synthetic resin. Such a primer is applied directly to the surface of a plastic part such as a polyurethane bumper which is cleaned in advance, and the primer coating film is solidified by drying and baking. After that a synthetic resin base top coat paint of a desired color is applied onto the primer film, followed by drying and baking. The primer film must be completely hidden by the top coat paint because every primer is inferior to top coat paints in weather resistance and chemical resistance though it is superior in adhesion to plastic surfaces.
Thus, it is necessary to employ a two-coat and two-bake process for accomplishment of monochromatic painting of a plastic article. In the case of forming a two-toned paint film it is necessary to employ a three-coat and three-bake process. That is, a top coat paint film of a desired first color formed on a primer film by the two-coat and two-bake process is masked in a desired pattern, and another top coat paint of a desired second color is applied, followed by drying and baking and then removal of the mask.
Due to indispensableness of first forming a primer coating film by the aforementioned steps including a baking step, the conventional painting process for painting articles formed of synthetic resins is complicated and time-consuming and entails high equipment costs.